As described in U.S. patent application Ser. No. 11/708,992 filed Feb. 20, 2007 entitled Wide Band Differential Amplifier Including Single-Ended Amplifiers Coupled To A Four Port Transformer and assigned to the assignee hereof, and incorporated herein by reference, it is only with difficulty that one can provide a wideband microwave differential amplifier. These amplifiers are used to suppress the common mode and allow for the passage of the differential mode. This patent application describes the utilization of transformers which are four port devices in which first and second single-ended amplifier circuits are coupled to a four port transformer circuit. However, the transformer described in this application is only capable of a 3:1 bandwidth, insufficient for a large number of microwave applications.
It is noted at lower frequencies, a conventional differential amplifier typically utilizes “long tailed pair” topology. With this topology, two input transistors each receive a respective input signal, with the transistors having their source or emitter terminals interconnected and connected to a constant current source.
As mentioned in this patent application this topology can often result in instability of the amplifier at higher frequencies and also is inadequate due to the fact that it can have a poor common mode rejection ratio. Poor common mode characteristics result in the amplifier being unable to adequately suppress signals present on both the inputs supplied to the input transistors. Moreover, at centimeter and millimeter wavelengths, wide band or broadband differential amplifiers having relatively constant gain for input signals over a wide range of frequencies are typically difficult to design when using the long tailed pair topology.
It will be appreciated that in differential amplifiers intermodulation distortion results from two input signals of different frequencies being mixed together due to non-linearities in the transfer characteristics of the device receiving these two signals. Note that mixing or intermodulation distortion of the two signals to a differential amplifier forms additional signals at frequencies that are at harmonic frequencies, or at combinations of integer multiples of either the frequencies of the input signals or their harmonics. This results in intermodulation products referred to as second order or third order intermodulation.
Also as mentioned in this application, a differential amplifier with good common mode rejection should suppress the even order intermodulation distortion signals. If the differential amplifier has poor common mode rejection, then these signals may not be adequately suppressed and may be present at the output of the differential amplifier. While these signals can sometimes be canceled out in narrow band applications, in wide band differential amplifiers intermodulation distortion signals are not filtered out and must be suppressed.
As described in the aforementioned patent application, the differential amplifier includes first and second single-ended amplifier circuits coupled to a four port transformer. The properties of the transformer which are sufficient to fully describe and specify the correct operation for use in said differential amplifier are also a subject of this invention. One type of component which meets these requirements is the so-called center tap transformer.
This having been said, the configuration described in the aforementioned patent application is limited in bandwidth to only a 3 or 4:1 bandwidth. This flies in the face of the requirement for a very high dynamic range broadband amplifier and also the requirement for directly interfacing to balanced antenna interfaces.
Note, most microwave differential amplifiers are made to operate primarily in the 1 to 20 gigahertz range, although the principals described herein apply to millimeter waves up to 100 gigahertz or higher.
More particularly, it is only with difficulty that differential amplifiers can be made to operate in the microwave range. If one could design a suitable wide band differential amplifier, it would allow one to accomplish the same type of tasks at the microwave range that can be done at audio frequencies or megahertz frequencies. However, these types of differential amplifiers have been difficult to implement at microwave frequencies. Particularly, isolation of signals from ground is virtually impossible with the microwave circuits presently in use; and this hinders the floating of signals. One of the reasons is the inability to provide a center-tapped transformer using MMIC technology. Note, center-tap transformers are normally not available utilizing MMIC technology which involves depositing circuits on planar surfaces. If one could have something other than a planar coil one might be able to provide a center-tapped configuration. However, such transformers are difficult to implement in an ultra small planar MMIC configuration.
One would also like to have a differential amplifier operating in the microwave region of the electromagnetic spectrum with a good common mode rejection ratio in which noise at the two inputs of the differential amplifier of the same amplitude and phase are suppressed. On the other hand, it is desirable that a differential signal at the two inputs be passed and amplified.
Thus, a true differential amplifier should have the capability to suppress second harmonic and second order distortion products, and there is a requirement to provide a differential amplifier that can do so. Also, it is important to provide such a structure for antenna interfaces which have balanced inputs.
It might be thought that one could utilize baluns to provide a balanced output from an unbalanced input. However, losses are associated with the use of baluns that greatly degrades sensitivity in the receive mode and also degrades overall power efficiency or output power capability in the transmit mode. If one could provide a true wideband differential amplifier, then one would be able to eliminate interfaces and circuit components that degrade the performance of the system.
As far as the microwave region of the electromagnetic spectrum, the above patent application describes separating the amplifying elements into something that can be independently designed as single-ended amplifiers and then recombining their outputs using a four port transformer to provide the differential amplification function.
While the amplifier elements can themselves be designed to operate over almost any bandwidth and any frequency, one could achieve almost arbitrarily wide bandwidth if one added a four port transformer element whose bandwidth was likewise unlimited. However transformers available utilizing MMIC technology have heretofore been limited to a 4:1 bandwidth.